1. Field of the Invention
The present invention relates to a receiver input circuit connected and disposed between a receiving antenna and a high frequency amplifier, and to a receiver input circuit which is equipped with an antenna impedance matching circuit and an LC parallel tuning circuit connected to the output side of the antenna impedance matching circuit and which is capable of attaining matching to antenna impedance without performing time-consuming adjustments, and obtaining a uniform frequency selective characteristic and a satisfactory noise figure.
2. Description of the Related Art
In a receiver that receives a high-frequency signal wave lying in a band of a few hundreds of MHz, ones of various circuit formats have been used as receiver input circuits thereof. However, the following are known as relatively well-used ones of the receiver input circuits.
In the first receiver input circuit, an inductor element with an intermediate tap is used as an inductor element that constitutes an LC parallel tuning circuit. A first variable capacitance type capacitor element is connected between the intermediate tap of the inductor element with the same and an antenna input terminal. When the tuning frequency of the LC parallel tuning circuit is adjusted by changing the capacitance of a second variable capacitance type capacitor element of the LC parallel tuning circuit, an adjustment to the capacitance of the second variable capacitance type capacitor element and an adjustment to the capacitance of the first variable capacitance type capacitor element are made together, thereby attaining the setting of the tuning frequency and matching to antenna impedance.
In the second receiver input circuit, as an alternative to the use of the inductor element with the intermediate tap as the inductor element that constitutes the LC parallel tuning circuit, an additive inductor element smaller in the number of turns than the above inductor element is inductively coupled to the corresponding inductor element. A first variable capacitance type capacitor element is connected between one end of the additive inductor element and an antenna input terminal. When the tuning frequency of the LC parallel tuning circuit is adjusted by changing the capacitance of a second variable capacitance type capacitor element of the LC parallel tuning circuit, an adjustment to the capacitance of the second variable capacitance type capacitor element and an adjustment to the capacitance of the first variable capacitance type capacitor element are made together, thereby attaining the setting of the tuning frequency and matching to antenna impedance.
Further, in the third receiver input circuit, a series-connected circuit of a first variable capacitance type capacitor element having a small capacitance value and a second variable capacitance type capacitor element having a large capacitance value is used as a capacitor element constituting an LC parallel circuit, as an alternative to the use of the inductor element with the intermediate tap as the inductor element that constitutes the LC parallel tuning circuit. A connecting point of the first and second variable capacitance type capacitor elements and an antenna input terminal are connected to each other. The tuning frequency of the LC parallel tuning circuit is adjusted by principally adjusting the capacitance value of the first variable capacitance type capacitor element having the small capacitance. Matching to antenna impedance is performed by principally adjusting the capacitance value of the second variable capacitance type capacitor element having the large capacitance. Therefore, the setting of the tuning frequency and the matching to the antenna impedance are attained by performing their adjustments together.
In this case, any of such known first through third receiver input circuits needs to carry out together, the adjustment to the tuning frequency of the LC parallel tuning circuit and the adjustment for the matching to the antenna impedance each time a signal wave received and selected by each receiving station is changed. Since, however, the adjustment to the tuning frequency and the adjustment for the matching to the antenna impedance cannot be performed under states independent of each other, respectively, there is a need to repeatedly perform these two adjustments on several occasions and thereby obtain their best points for the purpose of obtaining the best conditions for the two states. Therefore, the state of the receiver input circuit can finally be adjusted so as to be best if such two adjustments are repeatedly performed. When, however, signal waves from many receiving stations are received and selected one after another, there is a need to perform such two adjustments in an extremely short period of time each time the signal waves from the respective receiving stations are received and selected. An adjuster who performs these adjustments takes time and efforts or trouble over their adjustments excessively. This becomes a large burden on the adjuster.
With an increase in communication demand and the progress of broadening or wider bandwidth of a usable frequency wave, frequency allocations that respective specific frequency bands are used have recently been performed according to use objectives. There has thus been under the condition that many signal waves different in frequency exist within a frequency band narrower than the above bands. Therefore, it can be said that the execution of both the adjustment to the tuning frequency and the adjustment for the matching to the antenna impedance for every receiving selection of many specific stations where signal waves sent from the specific stations are selected by a receiver, is an extremely unrealistic means. Therefore, one for simply performing only the adjustment for the tuning frequency in the receiver input circuit without performing the adjustment for the matching to the antenna impedance in particular, or one for allowing a station selecting operation done by an LC parallel tuning circuit and a frequency selective characteristic to depend on a frequency selecting function of an intermediate frequency stage or the like lying subsequent to a frequency conversion stage without the use of the LC parallel tuning circuit by simply using a wideband bandpass filter in the receiver input circuit has frequently appeared in recent years.
However, in the receiver input circuit that performs only the adjustment to the tuning frequency without carrying out the adjustment for the matching to the antenna impedance, the matching to the antenna impedance is fixed to its matching approximate point, and a noise figure is considerably degraded although a characteristic to be almost satisfied can be obtained as to frequency selectivity. The receiver input circuit with no use of the LC parallel tuning circuit applies many frequency signals to the frequency conversion stage simultaneously by simply using the wideband bandpass filter. Therefore, image selectivity and an inter-modulation characteristic are deteriorated and an improvement in noise figure cannot be expected.
Meanwhile, a decision as to whether the noise figure is good or bad is performed based on grounds to be described below. That is, the noise figure of the receiver input circuit is generally expressed in the following equation according to “Electronic Information Communication Handbook”, published by Ohmsha Ltd., Institute of Electronics, Information and Communication Engineers, pp. 2398, 1988:F=1+(Rs/R0)+(RN/Rs){(1+Rs/R0)}  (1)
In this case, R0 is a termination resistor or resistance of the input circuit and indicates a parallel value of resonance impedance of an LC parallel tuning circuit and input impedance of a high frequency amplifier when the LC parallel tuning circuit is used. Rs is impedance as seen on the input side from an input point of the high frequency amplifier and indicates impedance other than the termination resistance R0. RN indicates an equivalent noise resistance obtained by converting all noise of a post-stage high frequency amplifier inclusive of a first-stage high frequency amplifier to an input terminal of the first-stage high frequency amplifier.
When antenna impedance and the input circuit are placed in an impedance-matched state, Rs is normally given as Rs=R0. A noise figure F (match) at this time is expressed as the following equation:F(match)=2+(4RN/R0)  (2)
The noise figure F (match) can be reduced as RN reaches RN<<R0 between the equivalent noise resistance RN and the termination resistance R0. The noise figure F approaches 2 corresponding to a final asymptotic value.
On the other hand, when the antenna impedance and the input circuit are in an impedance-unmatched state, the noise figure is expressed by the following equation assuming that, for example, an error of Rs=R0 (1±10%) exists between the impedance Rs and the termination resistance R0:F≈(2±5%)+(4±10%)(RN/R0)  (3)
When only the bandpass filter is used without using the LC parallel tuning circuit, R0=RA is established between the termination resistance R0 and the antenna impedance RA. When RA=50, the equivalent noise resistance RN may be considered to be RN>50. Therefore, the noise figure F is considered to be approximately 6 or more.
When the matching to the antenna impedance is adjusted simultaneously when the LC parallel tuning circuit is used in the high frequency input circuit, the noise figure F (match) is expressed in the equation (2) from above. When the matching to the antenna impedance is approximately carried out while the LC parallel tuning circuit is used, the noise figure F is approximately expressed in the equation (3). In either case, the larger the termination impedance R0 of the input circuit as compared with the equivalent noise resistance Rn, the more the noise figure F can be reduced. Thus, the noise figure F can be made close to the asymptotic value 2.
On the other hand, when only the bandpass filter is used in the high frequency input circuit without using the LC parallel tuning circuit, the noise figure F would be approximately 6 or more as described above.